1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for transmitting Precoding Matrix Indexes (PMIs) for Base Stations (BSs) included in a Coordinated Multi-Point (CoMP) set in a User Equipment (UE), when a wireless communication system using a CoMP scheme operates in joint processing mode.
2. Discussion of the Related Art
Multiple Input Multiple Output (MIMO) has recently attracted much attention as a broadband wireless mobile communication technology. A MIMO system seeks to increase data communication efficiency by use of a plurality of antennas. Depending on whether the same data or different data are transmitted through antennas, MIMO techniques are classified into spatial multiplexing and spatial diversity.
Spatial multiplexing is characterized in that different data are transmitted simultaneously through a plurality of Transmission (Tx) antennas. Therefore, data can be transmitted at a high rate without increasing a system bandwidth. In spatial diversity, the same data is transmitted through a plurality of Tx antennas, thus achieving transmit diversity. Space time channel coding is a kind of spatial diversity scheme.
Depending on whether a receiver feeds back channel information to a transmitter, MIMO techniques are also categorized into open-loop MIMO and closed-loop MIMO. Open-loop MIMO schemes include Bell Labs Layered Space-Time (BLAST) and Space-Time Trellis Coding (STTC). According to BLAST, the transmitter transmits information in parallel and the receiver detects signals by repeating Zero Forcing (ZF) or Minimum Mean Square Error (MMSE) detection. Thus as much information as the number of Tx antennas can be transmitted. STTC achieves transmit diversity and coding gain by utilizing space. Transmit Antenna Array (TxAA) is a closed-loop MIMO technique.
FIG. 1 conceptually illustrates a CoMP scheme applied to a wireless communication system under a multi-cell environment.
Referring to FIG. 1, there are intra enhanced Node Bs (eNBs) 110 and 120 and an inter eNB 130 in the multi-cell environment. An intra eNB covers a plurality of cells (or sectors) in a Long Term Evolution (LTE) system. Cells covered by an eNB to which a User Equipment (UE) belongs are in an intra eNB relationship with the UE. That is, cells covered by the same eNB that manages a cell in which a UE is located are intra-eNB cells, and cells covered by a different eNB from the eNB that manages the serving cell of the UE are inter-eNB cells.
Cells covered by the same eNB that serves a UE exchange information (e.g. data and Channel State Information (CSI)) through an x2 interface, while cells covered by a different eNB from the serving eNB of the UE exchange inter-cell information via a backhaul 140. As illustrated in FIG. 1, a single-cell MIMO user 150 located in a single cell (or sector) may communicate with one serving eNB in the cell (or sector), and a multi-cell MIMO user 160 located at a cell edge may communicate with a plurality of serving eNBs in a plurality of cells (or sectors).
CoMP was proposed to improve the throughput of a user at a cell edge by applying advanced MIMO under a multi-cell environment. The use of CoMP in a wireless communication system may reduce Inter-Cell Interference (ICI) in the multi-cell environment. Multi-cell eNBs may provide joint data support to a UE by a CoMP operation. Also, each eNB may improve system performance by simultaneously supporting one or more UEs (or Mobile Stations (MSs)) MS1, MS2, . . . , MSK. Further, an eNB may implement Space Division Multiple Access (SDMA) based on CSIs between the eNB and UEs.
There are largely two CoMP operation modes, joint processing mode which is cooperative MIMO based on data sharing and Coordinated Scheduling/Beamforming (CS/CB) mode.
In a CoMP wireless communication system, a serving eNB and one or more neighbor eNBs (or Base Stations (BSs)), BS1, BS2, . . . , BSM are connected to a scheduler over a backbone network. The scheduler receives feedback channel information representing channel statuses between the UEs, MS1 to MSK and the eNBs BS1, BS2, . . . , BSM, as measured by the eNBs. For example, the scheduler may schedule cooperative MIMO information for the serving eNB and the one or more cooperating eNBs. That is, the scheduler issues a command related to a cooperative MIMO operation directly to each eNB.
In a closed-loop wireless communication system, a UE may measure the channel quality of a data transmission channel between the UE and an eNB, select a PMI for the eNB, and transmit Channel Quality Information (CQI) representing the channel quality and the PMI to the eNB. When the closed-loop wireless communication system operates in CoMP, a UE may transmit CQIs and PMIs for eNBs included in a CoMP set to a serving eNB, for implementing a more efficient joint processing mode.
The PMI transmission from the UE in the joint processing mode may be considered in two ways. One is for the UE to select PMIs for the eNBs included in the CoMP set and transmit the individual PMIs to the serving eNB, and the other is for the UE to transmit a joint PMI for the eNBs included in the CoMP set to the serving eNB.
However, the current LTE standards have specified no criterions by which a UE receiving data in the same frequency band from eNBs included in a CoMP set selects a joint PMI for the eNBs and transmit the joint PMI. Accordingly, a joint PMI that a UE selects and transmits in the joint processing mode will be defined and exemplary embodiments of the present invention for using a joint PMI will be provided.